It is proposed to continue the genetic analysis of the P2 - P4 system consisting of two temperate bacteriophages, P2 (or a P2 relative) and P4, and their common bacterial host, Escherichia coli. In this system P2 (or its relative) serves as "helper" for its "satellite" P4. The main aim is to further elucidate the genetic control mechanisms that couple gene expression for satellite and helper. These include mutual derepression and transactivation. Investigations of other interactions characteristic for this system are also planned. Specifically the proposal concerns: (1) studies of P4 mutants able to use as helper a P2-related prophage not accessible to wild type P4, to identify the critical interaction for this helper - P4 combination; (2) studies of mutations affecting phage head size determination in the P2 - P4 system, especially further analysis of "shrink resistant" P2 mutants, also (3) related studies of P4 - plasmid interactions leading to the formation of transducing "phasmids"; (4) a search for P2 mutants deficient in reciprocal transactivation of P4; (5) further studies of the effects fo mutation in rpoA (the gene for he Alpha subunit of the bacterial RNA polymerase) on gene expression in the P2 - P4 systems; (6) studies of bacterial filamentation caused by P4 mutants present as prophage or infecting phage; (7) further studies of P2 cox mutants and their effects on derepression and recombination; (8) further studies of suppresor mutations that compensate forlack of an early P2 gene function and; (9) a search for P4 related genomes and their eventual analysis.